Increase of the enclosed mass for several orbits for our time-dependent potential with a growth factor of a_g = 0.8. The dashed lines show the enclosed mass within the final value of their apocentre. Since the apocentric distances shrink with time, we show the mass enclosed within the time-dependent apocentre values with solid lines. The integration time span for the streams is t ~ 7–8 Gyr, and the enclosed mass approximately doubles over this timescale.